Fixed subscription database cache out of sync

The subscription reloading from database could cause cache
to be out-of-sync. Fixed by making reloading synchronous and
only reload when necessary (e.g. Backup restore actually changes
the database).

Fix: 290295550
Test: atest SubscriptionManagerServiceTest TelephonyProviderTests
Test: Manually test backup/restore
Test: Telephony basic functionality tests
Test: Re-tested passed on b/290295550#comment#23
Change-Id: Iee0355a4069ce6d5c85ecc1bd727bc457c3bb902